How can messages be transmitted secretly? How can one guarantee that the message arrives safely in the right hands exactly as it was transmitted?

Cryptology -- the art and science of "secret writing" -- provides ideal methods to solve these problems of data security.

Cryptology is divided into two branches -- cryptography and cryptanalysis.

Cryptography is the science which embraces all the methods and devices whereby an intelligible, written message may be converted into an unintelligible or secret form.

Cryptanalysis, on the other hand, is the science which embraces all the principles, methods and means employed in the analysis of secret messages (cryptograms), i.e. their reduction or solution without the knowledge of the system or the key or the possesion of a code book.

There are many methods to encrypt a message. One of the methods is to encode, i.e. to replace a message's elements by an arbitrary combination of characters or figures. To encipher a message is to apply a method of cryptography to the individual letters of the plain text, taken singly or in groups of constant length. Here, we will deal not with codes, but with ciphers.

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